For use in powering a vehicle, an internal combustion engine must react to a variety of operating conditions to effectively provide desired performance. A technique to achieve such desired performance involves a system to selectively adjust the timing or phasing of the opening and closing of the engine's valves relative to rotation of the engine's crankshaft. Selective control of the intake valves is particularly effective. Likewise, particularly as concerns diesel engines, selective control of the timing of a fuel injection pump offers distinct advantages.
Known devices include sections of two rotating shafts being machined with splines with at least one of the shafts having helical splines. Another connecting shaft or shifting sleeve engages one rotating shaft with the other rotating shaft and has two sets of splines designed to mesh with the respective rotating shafts. The relative rotation between the rotating shafts and the connecting shaft may take place simultaneously with otherwise high speed rotation of the entire assembly under engine load conditions.
In known systems of the type previously discussed above, the shifting sleeve is typically activated by a large piston which may be coaxially mounted relative to the shifting sleeve. This piston is moved in a first axial direction by a force created by application of pressurized engine oil as directed to one end of the piston by control of a solenoid operated valve. When the force produced by the pressurized oil is absent, a return spring typically biases the piston in a second opposite axial direction. Thus, the aforedescribed known mechanism basically produces a two-position timing or phasing function. This means that the system is not capable of selectively producing a stable, intermediate position between the first and second end positions of the piston. The inability to achieve such a stable intermediate position compromises optimal valve timing of an engine. If the aforedescribed system attempts to achieve an intermediate position, the return spring provided to bias and return the piston to the first position is subjected to cam torque impulse forces which undesirably produce unintended changes in the timing or phasing.
Secondly, undesirable consequences can result from the use of pressurized engine oil as the actuating force for operating the prior variable valve timing systems described above. The rotation of the two shafts, including the camshaft and the piston actuator, tends to act as a centrifuge with respect to the oil and resultantly separate solids in the oil from the remaining liquid portion. These solids can be deposited adjacent the periphery of the actuator piston and accumulate sufficiently to eventually interfere with the smooth reciprocal operation of the actuator piston. Furthermore, seals for containing the oil will wear and then the mechanism likely will develops either internal or external oil leaks. In addition, operation of this type of oil pressure actuated mechanism under cold conditions can be very sluggish due to the high viscosity of the oil.
Ideally, a compact variable timing device or mechanism that selectively provides infinite timing or phasing changes is sought. Such a mechanism would be desirable for timing an engine camshaft with respect to the engine's crankshaft for achieving a true variable valve timing system. The ideal mechanism would also be desirable for timing or phasing an input shaft of an engine distributing type fuel injection pump as is commonly used for diesel or other direct injection type engines, thus providing selectively timed fuel injection. In either application, what is needed is a shaft timing mechanism that can be either retarded or advanced relative to the crankshaft in a precise fashion in response to operating conditions of the engine.